What's the best way to bring down revs at cruise speed?

[edgyWedgy]

Hi,
I just finished renovating my '76 TR7, and i have driven 150ml/250km with it so far, (without any problems i can proudly say!:)
but it's equipped with a 4 speed and the standard 3.63 diff, and the revs are too high for my liking.
The noise is not that much of a problem, but the fuel consumption and the reduced engine lifetime are.
I use the car to commute and drive to school and other everyday acitivities, so i'll put a lot of miles on the odometer,
and at higher speeds also. (in the Netherlands the speedlimit on highways is 130kmh/81mph)

While cruising at 120kmh/70mph revs are at 4000 already, while i'd like them to be around 2500 at this point.
I was wondering what would be the best way to achieve low rev cruise speeds.
I guess the cheapest way would be to find a junk TR7/8 with a complete 5 speed drivetrain, but there aren't many TR's available in my region, let alone 5 speed equipped ones.
I also have a few questions which i hope you'll be able to answer:
-Are there are other/better 5 or even 6 speed gearboxes that fit on either of the two available bellhousings?
-Is it possible to rebuild the early diff to a different ratio?
-Is it recommendable to use a diff with a significant lower ratio like the TR8's 3.08 diff?
To lose some torque on the wheels would not be that of a problem for me, since the TR7 has enough torque.

All help and tips would be welcome, cheers!

[FI Spyder]

With 5 speed and 3.90 diff. I think I'm turning about 3300 rpm at 70 mph. While a little noisy at first you get used to it soon and it doesn't bother you. Fuel mileage seems to directly related to speed (wind resistance) and not directly to rpm except that at higher rpm there is more internal friction therefore less efficiency. I have gotten over 40 miles per (imp) gallon at 50 mph but it goes down directly as wind resistance (speed) increases. Changing the ratios might help a little, the later/last TR7's had a 3.46 diff and some had a higher 5th gear in an attempt to increase gas mileage. Some of that is lost because as the mechanical advantage decreases you need more gas pedal to over come wind resistance (engine has to work harder). I wouldn't worry to much about high rpm wearing out the engine. I read a post were a guy drove his 180,000 before he had to touch the engine and that was in LA traffic where you can bet he was turning a whole lot more rpm per mile than we will (getting stuck in traffic jams, you turn a lot of rpm's per mile) plus today's oils are a lot better than back then (1995). Going to a 3.08 diff would make the engine work a lot harder. It would be interesting to see what that combo would be like though.

[john 215]

Hi,

I ran a four speed axle with a 3.27:1, this is found in automatics up to a certain chassis no, then they went 5 speed axle. It was a little tall if the truth be said so acceleration was not exactly brisk !

You can use a 5 speed axle with a four speed box BUT will require a one off propshaft and ideally different bottom arms.

I run a overdrive box in my four speeder, out of a Dolomite 1850, they are rare and expensive over here so guess unattainable where you are !

Cheers John

[Beans]

Best standard solution to bring the revs down is fitting the 5-speed 'box with a 3.45 axle.
On standard size wheels/tyres it'll cruise happily at 120 km/h @ 3000 rpm.
But acceleration won't be as crisp as with the normal 5-speed axle ratio of 3.90:1 combination.

[Hasbeen]

The standard 5 speed gives you 33.5K/H/1000 RPM in 5Th. On a trip with 75% cruising at 3300 RPM, [70Km/H] & 25% town running my 7 returns a bit under 8L/100Km. On a Triumph club run through the ranges with enthusiastic driving this can climb to a bit over 10L/100Km.

If you could find one of the 3.45 rear axles it would suit you well. The 3.08 from the TR8 would be ridiculous.

Be it wind resistance or simply engine revs, I have found the difference over a few hundred kilometres between 90Km/h & 110 Km/h is about 2 L/100Km. Fuel wise our cars prefer to run a bit under 3000 RPM.

Hasbeen

[edgyWedgy]

@FI Spyder
180k is a lot on such an old engine, i'd say! Did he modify his engine? If not, i see why i shouldn't worry about the revs, regarding engine lifetime. And if i also could achieve 40mpg that would be really great. I estimated the engine would run 25mpg.
I feared the 3.08 diff would be too heavy, but changing the diff would cost a lot less money, and it's still possible to upgrade to a 5 box later.
That's also possible with the 3.45 diff, so i think i'll do that.

[edgyWedgy]

@john 215
Would a dolomite 1850's gearbox with overdrive fit? that could also be a solution.
They are available, at €1200. But i suppose that you have to shorten the propshaft to fit, and maybe even mount a different flange.
I'm curious what did you have to modify to make it work? Are there also other gearboxes you know that fit?

@Hasbeen
That's some useful information, about fuel consumption around 3000rpm.
My guess would be the revs, since the shape looks quite aerodynamic. I'm working on drawing the TR7 in SolidWorks CAD to measure the drag coefficient. I'll keep track of my fuel consumption to see if it works for me also.

[FI Spyder]

The 180K (may have actually been 190K and it was the transmission that needed overhaul at 180K) was on an unmodified engine. When you modify it you are going to shorten the lifespan. It's not unusual to get high mileage out of old engines. There are a number of ways to kill an engine (shorten it's life span) but if treated properly they will last a long time. Example: Short trips (like taking the kids a few blocks to school or to the nearby shopping centre) are an engine killer. People who drive on long trips tend to pile on more miles and their cars last longer, all other things being equal.

[jeffremj]

Hi,

I ran a four speed axle with a 3.27:1, this is found in automatics up to a certain chassis no, then they went 5 speed axle. It was a little tall if the truth be said so acceleration was not exactly brisk !

You can use a 5 speed axle with a four speed box BUT will require a one off propshaft and ideally different bottom arms.

I run a overdrive box in my four speeder, out of a Dolomite 1850, they are rare and expensive over here so guess unattainable where you are !

Cheers John

I used to have a Dolomite 1850 without overdrive. To give it more 'legs' I put in the Dolomite automatic axle (3.27:1). Unfortunately, it only lasted 12 months before it 'blew up'.

Note that the axle centre assembly of the automatic Dolomite is the same as for the early auto TR7 - TKC2768.

It seems that Morris Marinas used the same axle, here are some details from the web:

Ratio Pinion/Crown Triumph Marina Revs@60mph Used in
4.55 9/41 217082 3954 1.3 Marina 7cwt Van from chassis No 22022
4.11 9/37 UKC1492 217081 3572 1300 Dolomite / 1.3 Marina Saloon and 7cwt van to car No 22021
3.89 9/35 UKC1493 3459 1500 Dolomite / 1.3 Ital
3.63 11/40 TKC2619 21H5549 3154 1850 Dolomite Manual / 4sp TR7Manual / 1.8 Marina/ 1.7 Ital Saloon & Estate
3.27 11/36 TKC2768 2842 1850 Dolomite Auto / 4sp TR7 Auto / Ital 2.0 Auto

So, search out an old Ital auto !!

[edgyWedgy]

@FI Spyder
Yeah indeed that's a known fact, i already drilled holes in the silencers to let out condensated water from short trips.
I'll use the car to commute 35km to work, so thats a moderately long trip.

@jeffremj
my current diff is 3.63 so the 3.27 would make a little difference.
I think i'll combine that diff with a 5 'box once i have saved up for the costs.
They are rather expensive though, so i'm seeking for another gearbox that will fit, but costs less money.

[FI Spyder]

Short trips and water condensation only affects the exhaust. Short trips prevents the contaminates in the oil from "boiling" off so it becomes acidic. That's why they suggest you change oil more often if you do short trips but that's just trying to mitigate the problem, not solve it.

[edgyWedgy]

Yeah but that's one thing i knew, and it's relatively easy to do. I also use the car for shorter trips.
Are there more measurements, other than changing oil early, that i can take to prolong engine life?
Other than early oil changes i've also heard it's recommendable to use oils with more phosphate and zinc additives.(mostly kendall oils)
I don't know if that's also applies with older engines though. What oil do you use by the way?
In moderate climates the manual recommends 15W40 oil, but i've seen people using 20W50 oil instead.

[Beans]

... I think i'll combine that diff with a 5 'box once i have saved up for the costs
They are rather expensive though, so i'm seeking for another gearbox that will fit, but costs less money.
In moderate climates the manual recommends 15W40 oil, but i've seen people using 20W50 oil instead ...

Unless you have access to proper machining equipment the 5-speed will be the more cost effective solution, even here in the NL.
There are plenty around.

As for the oil, always use 20W50 oil all year 'round. With that inside a friend managed 300.000 km on one engine in 10 years.
And that with only the most basic maintenance.
I have been using Valvoline VR1 Racing for many years now and both my TR7's are still going strong.

[edgyWedgy]

I have access to a turning lathe and a router, so i can make a flange to fit the propshaft/diff/'box.
The LT77 is sold at Nico Baas here in NL, but it's rather expensive at €1000, for me though as a student.
And i believe the gearshifter body isn't even included, so that'll make for another €200 at least.

Just to know what's available, i'm looking around for other gearboxes that may fit, but with other ratio's.
I'm planning to put the TR7 on de dyno bench at school to find out how torque and power develop over rpms.
(i'll post the result here for you guys to see.)
Then the best solution would be to have a gearbox that falls back to maximum torque rpm when you shift at maximum power rpm, or at least one that comes the close to that. Maybe it'd be a good idea to look for a junk TR7 with a 5 'box, but i guess there aren't many around in NL or Germany.

As for the oil, does 20W50 significantly increase fuel consumption? If not, i'd choose 20W50.
I saw your blog about your TR's they look amazing by the way!

[John_C]

My four speed runs about 3700 at 70mph. Never could figure why it's different to others. But the 70mph is dead on as it matches GPS exactly.